With the recent increase in the capacities of optical transmission systems, modulation rates and the scale of configurations of optical transmission devices equipped with optical modulators are increasing. Therefore, it is desirable to reduce the size of an optical modulator by integrating a plurality of Mach-Zehnder type optical waveguides (hereinafter, described as “MZ optical waveguides”), which perform optical modulation, into an optical waveguide group on a single substrate.
In an optical modulator in which a plurality of MZ optical waveguides are integrated into an optical waveguide group on a single substrate, the plurality of MZ optical waveguides are provided in parallel on the substrate. Specifically, a pair of relay optical waveguides branch from an optical waveguide for inputting light, a pair of first MZ optical waveguides are connected to the pair of the relay optical waveguides, and a pair of second MZ optical waveguides are provided on a pair of branched waveguides (hereinafter, described as “arms”) of each of the first MZ optical waveguides. However, in an optical modulator in which a plurality of MZ optical waveguides are provided on the substrate, the size in a longitudinal direction of the substrate is increased in order to accommodate the lengths of the pair of the relay optical waveguides and the lengths of the pair of the arms of each of the first MZ optical waveguides.
Incidentally, there is a known structure in which input ends of a plurality of MZ optical waveguides (i.e., the pair of the first MZ optical waveguides and the pair of the second MZ optical waveguides) are arranged at positions that deviate from one another in the longitudinal direction of the substrate. If the input ends of the plurality of MZ optical waveguides are arranged at positions that deviate from one another in the longitudinal direction of the substrate, the lengths of the pair of the relay optical waveguides and the lengths of the pair of the arms of each of the first MZ optical waveguides are reduced to some extent; therefore, it becomes possible to reduce the size in the longitudinal direction of the substrate.    Patent Literature 1: Japanese Laid-open Patent Publication No. 2010-185978
However, with the structure in which the input ends of the plurality of MZ optical waveguides are arranged at positions that deviate from one another in the longitudinal direction of the substrate, while it is possible to reduce the size in the longitudinal direction of the substrate, it is difficult to reduce deterioration of an optical property, such as an extinction ratio.
Specifically, with the structure in which the input ends of the plurality of MZ optical waveguides are arranged at positions that deviate from one another in the longitudinal direction of the substrate, the pair of the second MZ optical waveguides provided on the pair of the arms of each of the first MZ optical waveguides are arranged at asymmetric positions. Therefore, a difference in optical losses occurs between the pair of the arms of each of the first MZ optical waveguides, and, as a result, an optical property, such as an extinction ratio, of the entire optical modulator may deteriorate.
According to an embodiment, an optical modulator disclosed in the present application includes a substrate and an optical waveguide group provided on the substrate. The optical waveguide group includes a pair of relay optical waveguides, a pair of first Mach-Zehnder type optical waveguides, and a pair of second Mach-Zehnder type optical waveguides. The pair of the relay optical waveguides branch from an optical waveguide for inputting light. The pair of the first Mach-Zehnder type optical waveguides are connected to the pair of the relay optical waveguides and includes input ends arranged at positions that deviate from each other in a longitudinal direction of the substrate. The pair of the second Mach-Zehnder type optical waveguides are provided on a pair of branched waveguides of each of the first Mach-Zehnder type optical waveguides and includes input ends arranged at symmetric positions with respect to a straight line that passes through an input end serving as a branch point of the branched waveguides and that extends in the longitudinal direction of the substrate.